A picture speaks 1000 words, etc.
Have you looked at the back of the door card? When they get removed people often take the plastic membrane off the door. Then when it rains/ any water goes on the window it pisses down the inside and the back of the door card and sits on the bottom and rots it, the door card is only made of reinforced card type stuff so it doesn't take long to rot through and end up like wet cardboard. Could be that, so it's just peeled off the door card due to the card being knackered.
They sit in little guides too (Golfs do, struggling to remember about polos) and they can bend/ weaken and make the face of the door card peel off.
If it is knackered then something like letting it dry naturally and pasting a shit load of PVA onto the back (so it soaks in- may need to water it down, a la 5 year old school project stylee) and letting it soak in/ dry should make it firm. Then get some glue to redo the front? Or buy vinyl wrap and heatgun it on over the top.
OR just buy another door card, that's what I'd do
